It is known to form a sheet of metal into a variety of shapes by means of a press such as a draw die tool. An example of such a tool is shown schematically at 1 in FIG. 1.
The tool 1 is supported on a base 1B and has an upper die member 10 located vertically above a punch member 20. Opposing surfaces of the upper die member 10 and punch member 20 have complementary shapes corresponding to that of the panel to be pressed therebetween. The upper die is of female type, defining a cavity into which a sheet of metal 5 that is to be pressed may be urged by the punch member 20 in order to press the sheet 5 into the desired shape.
The sheet 5 is arranged to be supported on a blank holder 30 that substantially surrounds the punch member 20. The tool 1 is arranged such that the upper die member 10 may be lowered towards the punch member 20 from the position shown in FIG. 1 so that the sheet of material 5 is gripped between the upper die member 10 and the blank holder 30. The upper die member 10 is then pressed further downwards, towards the punch member 20, bringing the sheet of material 5 into contact with the punch member 20. The sheet 5 becomes sandwiched between the upper die member 10 and punch member 20 and is forced to bend into conformity with the complementary shapes thereof as the punch member penetrates the cavity defined by the upper die member 10. The punch member 20 is forced into the cavity until the sheet 5 is drawn to a shape corresponding to that of the lower punch member 20.
In the tool 1 shown in FIG. 1, the lower punch member 20 is supported on the base 1B by resilient spring elements 22. The blank holder 30 is supported on the base 1B by dampers 32.
It is to be understood that the process described above for pressing sheets of metal is ideally suited to a mass production environment since it allows panels to be shaped in a relatively rapid, reproducible manner.
It is known that, for a given sheet of material, a minimum radius of curvature may be defined below which witness marks are formed on the surface of the sheet when the sheet is pressed between the upper die member 10 and punch member 20. Witness marks are unsightly and evidence of their presence is visible through paintwork. Panels bearing witness marks require a substantial amount of finishing following the pressing operation before they are suitable to be used in a vehicle. Witness marks or witness lines are believed to be caused by excessive stretching of the sheet.
In some arrangements, if the radius of curvature is below a certain value, an amount of shear stress induced in the panel during pressing may cause tearing of the panel.
It is desirable to be able to manufacture panels of reduced curvature in a rapid and reproducible manner suitable for incorporating into mass-produced vehicles with little or no post-pressing finishing required.